I. Field of the Invention
The present invention relates to an improvement of surface treatment for an iron-based permanent magnet including rare-earth elements.
More particularly the invention presents a permanent magnet exhibiting high resistivity against mechanical attack, thermal attack and/or chemical attack, as well as comprising superior magnetic properties.
II. Description of the Prior Art
Permanent magnet materials are very important electric and/or electronic materials to be incorporated in various equipment such as consumer electronic equipment used in homes or in a peripheral terminal device for a large scale computer.
Recently, in an attempt to reduce size and enhance efficiency of permanent magnet materials have been sought.
As a new type of high efficiency magnet which does not include highly expensive samarium and cobalt, a R-Fe-B type (R: one element or a mixture of rare-earth elements including yttrium) was proposed.
This type of magnet exhibits an extremely high energy product value, (BH)max of 25 MGOe or more. The magnet contains light rare-earth such as Nd and/or Pr and Fe as its main constituent element.
A sintered magnet of Nd-Fe-B type is disclosed in Japan Laid Open Patent Application 59-46008. A bulk magnet of Nd-Fe-B type made from rapidly quenched metal flakes (Rapidly Quenched Type of Magnet, hereinafter) is disclosed in Japan Laid Open Patent Application 60-9852.
Although these magnets exhibit magnetic anisotropy and have a high energy product of 25 MGOe or more, they are inferior to samarium-cobalt magnets with respect to corrosion resistance, because these magnets include rare-earth elements and also iron, both of which are easily oxidized and form red rust on their surfaces. The efficiency of the magnet and the uniformity of magnetic properties will decrease when these magnets suffer corrosion.
In order to prevent corrosion of these Nd-Fe-B type of magnets, various methods have been proposed to produce a corrosion resistive film on the surface of these magnets.
However, the reliability of these corrosion resistive films is reduced when the adhesion of the corrosion resistive film to the magnet surface is not sufficient, even if the corrosion resistive film still covers the surface of the magnet. It is important to firmly fix the corrosion resistive film to the surface of the magnet.
It seems that no effective improvement was proposed previously to make the adhesiveness of the film to the magnet body stronger by improving the alloy composition itself.